As one of the noninvasive methods for the administration of drugs, a drug is administered with the use of percutaneously absorbable preparations. For instance, percutaneously absorbable preparations such as ointment, cream, lotion, poultice and patch have been used. The use of these percutaneous preparations is usually limited to the local therapy of disease localized on the skin. Because of the barrier function possessed by the skin, the systemic therapy with drugs through the percutaneous route is difficult due to the low systemic availability of locally applied drugs to the skin. Though several patch type Transdermal Therapeutic Systems (TTS) have been launched onto the market, the drugs are limited to estrogen, nitric acid derivatives, tulobuterol and nicotine etc. that show their pharmacological activities at low plasma or serum concentration, i.e., their therapeutic concentrations are lower than 20 ng/mL. So far, the absorption of macromolecular drugs such as insulin through percutaneous route is difficult because of their low skin permeability and no percutaneous preparations have been developed up to now. Therefore, macromolecular drugs are still administered to patients by injections.
Under such a background, development of injection technology with low invasion has been challenged and microneedle was developed as one of those technologies. Microneedle is a fine needle having no pain when applied onto the skin. As microneedle material, not only steel as a conventional injection needle but also silicon etc, are used (Non-patent documents 1 and 2). These microneedles have holes in themselves as conventional injection needles and drug solutions are injected through these holes. In addition, self-dissolving microneedle made of base material dissolving in the body was also developed. Active substance is contained in the base and is released from microneedle by the dissolution of the base after inserted into the skin. For instance, a self-dissolving microneedle made of maltose as the base is disclosed (Patent document 1). In addition, the self-dissolving microneedles made of polylactic acid, polyglycolic acid or poly-ε-caprolactone are also known.
In addition, when the active substance is a drug that receives high clearance from the systemic circulation such as insulin, long-term pharmacological activity is needed. In such a case, self-dissolving microneedle having the function of sustained-release characteristics of the active substance is required. For instance, a self-dissolving microneedle consisting of polylactic acid has a sustained-release function of the active substance.    Patent document 1: JP2003-238347 A    Non-patent document 1: D. K. Armini and C. Lui, “Microfabrication technology for polycaprolactone, a biodegradable polymer”, Journal of Micromechanics and Microengineering, 2000, 10, 80-84    Non-patent document 2: M. R. Prausnitz, “Microneedles for transdermal drug delivery”, Advanced Drug Delivery Reviews, 2004, 56, 581-587